The present invention generally relates to circularly polarized (CP) radio antennas and, more particularly, to an antenna comprising at least two pairs of crossed dipole antennas.
Conventional CP radio antennas in a crossed-dipole or xe2x80x9cturnstilexe2x80x9d configuration are well known in the art. An exemplary conventional CP radio antenna includes crossed dipole antennas fed by a balanced four-phase transmission line and located above a reflecting screen. Its dipole legs of the crossed dipole antennas incline downward toward the screen in order to increase the CP radiation at lower elevation angles relative to the plane of the screen. Antennas of this type can be constructed using simple wires, rods, or printed conductors for the dipole legs. A CP radio antenna having the above discussed features is depicted in FIG. 28-7 of the 3rd edition of the Antenna Engineering Handbook, published by McGraw-Hill, relevant portions of which are incorporated herein by reference.
In U.S. Pat. No. 5,519,407, a CP dual frequency antenna is described. This CP antenna includes four identical antenna elements each of which includes an inductor-capacitor trap positioned along the length of each antenna element. This configuration permits the disclosed CP antenna to operate at two different frequency bands.
Furthermore, in U.S. Pat. No. 5,526,009, a linearly polarized (LP) dual frequency antenna is described. This LP antenna includes an antenna assembly that comprises four antenna elements. Each antenna element includes a coil and an elongated arm. Pairs of the elongate arms form dipoles which are of differing lengths so that each pair of antenna elements resonates at a different frequency.
The present invention provides a nested turnstile antenna structure capable of transmitting and/or receiving CP electromagnetic waves in more than one frequency band. The antenna of the present invention also has a capability to achieve desired elevation radiation patterns within each frequency band.
The present invention is preferably used in reception of CP signals from Global Positioning System (GPS) satellites, and for transmission and reception of L-band communications satellite CP signals (e.g., signals used in the International Maritime Satellite System (INMARSAT) service), but it is not limited to use with above-discussed systems. For instance, the present invention may also be used for multifrequency communications using CP signals, for which omnidirectional, elevation-tailored radiation patterns are required.
In the present invention two or more turnstile antenna structures share a common symmetry axis and common reflector. Various design characteristics (e.g., lengths, positions along its symmetry axis, inclinations to a reflector and like) of radiating elements of crossed dipole pairs are preferably selected to achieve the aforementioned radiation characteristics.
In particular the present invention provides a circularly polarized multifrequency antenna. The antenna includes a reflector having a first side and a second side, a first crossed dipole pair having a first resonant frequency and a second crossed dipole pair having a second resonant frequency. The first and second dipole pair are symmetrically disposed on the first side of the reflector.